There are lots of contributing factors, many of which others have already mentioned.
One reason, I think, is that people who buy a Nintendo console are, more often than not, platforming fans. Fans of most other genres will likely look to the PS3/360 first, but for platforming fans, there really is only one choice. People talk about the 360 being shooter-focused, or at least the fanbase leans in that direction, but the core gamers for the Wii likely follow that same kind of pattern.
Nintendo also like to avoid continuing stories, and I think this is part of the reason for that. This way, it doesn't matter when you enter a series. There is never the feeling that you've spoiled the plot or that you know too much to enjoy what came before. Keep the games as rough copies of one another and you can keep producing them forever.
